Along with the trend toward finer semiconductor devices, it will be required in the near future to form trenches or the like with fine pitches of, e.g., about 30 nm, on an insulating film or the like in a semiconductor device manufacturing process. Thus, a plurality of line-shaped portions (lines) needs to be formed with fine pitches on the mask film.
In a conventional lithography using a laser beam or the like, a minimum width that can be developed is about 50 nm. Therefore, it is difficult to form a plurality of lines with pitches smaller than about 50 nm on a photoresist film serving as a mask film.
To that end, a double patterning process has been recently developed as a technique for forming a plurality of lines with fine pitches on a photoresist film. A representative example of the double patterning process includes: forming a base film, an interlayer film and a first resist film on a substrate; forming a first resist pattern by performing first exposure; forming a first interlayer pattern by transferring the first resist pattern onto the interlayer film; forming a second resist film on the first interlayer pattern; forming a second resist pattern by performing second exposure; and forming a second interlayer pattern having a plurality of lines formed with fine pitches by transferring the second resist pattern onto the interlayer film (see, e.g., Japanese Patent Application Publication No. 2008-258562 and corresponding US patent Application Publication No. 2008-193382 A1).
As for a mask film, a hard mask film made of a silicon-based material such as silicon carbide (SiC) or the like may be used instead of a photoresist film. In the representative example of the double patterning process using the hard mask film as shown in FIGS. 7A to 7E, an organic film 71 having a plurality of lines 71a formed on a processing target film 70 is etched such that a width of each of lines 71a is reduced to about 30 nm (FIG. 7A). Next, the lines 71a and the processing target film 70 are uniformly coated with a silicon oxide film 72 by CVD (Chemical Vapor Deposition) (FIG. 7B). By CVD, the lines 71a grow into thicker lines 72a. Then, the silicon oxide film 72 between the lines 72a and on the lines 72a is removed by anisotropic etching (FIG. 7C).
Thereafter, each of the lines 72a constitutes a pair of lines 74a and 74b by removing the exposed organic film 71 in the lines 72a by ashing (FIG. 7D). The pitch between the lines 74a and 74b of each pair is substantially equal to the width of each of the lines 71a, so that the pairs of the lines 74a and 74b can be used as a mask film having lines formed with pitches of about 30 nm.
However, when the organic film 71 is removed, ions that are not perpendicularly incident toward the processing target film 70 are introduced into spaces 73 formed by removing the organic film 71 of the lines 72a, so that the spaces 73 are enlarged except for upper portions thereof. As a consequence, each pair of lines 74a and 74b forms a pair of asymmetric sidewalls having a crab claw shape (FIG. 7D). Specifically, an upper end portion of one line is bent toward the other line. Then, openings 75 are formed in the processing target film 70 by etching the processing target film 70 while using the each pair of lines 74a and 74b (formed of the silicon oxide film 72) as a mask. At this time, some of the ions entering the spaces 73 may be reflected in unexpected directions (indicated by an arrow in FIG. 7D) by collision with the bent end portions of the lines 74a and 74b, so that some of the ions may collide with the processing target film 70 in non-perpendicular directions. As a result, the cross sectional shape of the openings 75 does not make a regular rectangular shape, but a rectangular shape having a swollen middle portion (FIG. 7E).